The long-term goal for this project is to elucidate the molecular mechanism for the regulation of programmed cell death. Understanding the mechanism is the key to the development of effective treatments for a wide variety of human diseases such as cancer as ischemia, in which apoptosis is either inhibited or accelerated. The fundamental question we will address is how Bcl-XL and Bax can have opposite functions (anti- and pro- cell death, respectively) yet display similar structures in solution. The specific aims are to determine (1) the structural basis for tBid-activated oligomerization of pro-apoptotic Bax in the mitochondrial outer membrane; and (2) the structural basis for inhibition of tBid-activated Bax by Bcl-XL. To complete the project, we will study the interactions between tBid, Bax and/or Bcl-XL using site-specific cross-linking, chemical labeling and fluorescence techniques. We will also generate mutations in these proteins to elucidate the structure-function relationship. By completing this systematic and thorough investigation, we will generate useful structural information about the membrane-embedded protein complexes, which may facilitate rational design of chemical inhibitors to target either the anti- or the pro-cell death complex for therapeutic benefits.